used a E3 plug on an 8hp Techumseh (4-stroke L-head) for a snow blower.
it was to replace Champion #J8C. it fouled up way to quickly (not even an hour of running). it didnt match the Champion plug properly (as it said it did - the E3 was a cold plug, Champ.#J8C is a hot plug).
replaced that with an Autolite plug and havent had any problems.
also got an E3 to replace Champ.#CJ8 (the cold version of J8C) on a 3.5hp Briggs L-head for a mower, but i put it on last fall when i rebuilt the engine. havent run it enough to know how it performs in that application yet. (the E3 in this case is a cold plug also, so i dont think i will have the same problem. but I'll find out when mowing season comes around)